We studied demography, territoriality and social system of the Brazilian population of Yellow Cardinal, Gubernatrix cristata . The study was conducted in the municipality of Barra do Quarai, western Rio Grande do Sul, Brazil, during two breeding seasons (October to February, 2013–2015). We ringed 35 (seven females, 14 males and 14 young) out of 53 individuals found, and by September 2015, the Brazilian population of Yellow Cardinal had 38 remaining individuals. Adult sex ratio was 1.5:1 and the lifespan was at least 8 years. Pairs are socially monogamous and territorial, with male-biased philopatry and female-biased dispersal. The mean territory size was 18 ha and mean home range was 27.7 ha. Some pairs and nests (23%) were attended by one or two nest helpers, which contributed on nest and territory defense, and provisioning of nestlings and fledglings. Presence of helpers is a vital new founding for this species. Parental care was biparental or cooperative, and the pair had high frequency of visits to the nest than helpers. Mean nest productivity was two fledglings/successful nest attended by helpers, and one fledgling/successful nest not attended by helpers. We recorded second broods after a successful attempt, only when helpers were present. Overall Mayfield nesting success was 18%, 40% for nests with and 13% without helpers. We recorded a case of inbreeding, between father and daughter. The species need for large home ranges and factors that may adversely affect the breeding success (e.g., inbreeding, predation, and parasitism) can aggravate the status of the Brazilian population and species. It reinforces the importance of autecology studies and their contribution to the conservation schemes.